The United States and Russia on Saturday reached agreement to eliminate Syria’s chemical weapons, giving President Bashar Assad one week to reveal what kind of weapons his country has and where they are being kept.

The agreement also calls for what one U.S. official called an “ambitious” timeline for dealing with Syria’s chemical weapons, setting a November deadline for eliminating that country’s ability to manufacture and mix the weapons and calling for the destruction of all materials that could be used to make such weapons by the middle of next year.

Under the agreement, inspectors from the Organization for the Prohibition of Chemical Weapons, the international body that monitors compliance with chemical weapons bans, would have “immediate and unfettered access to inspect any and all sites in Syria.” Their initial inspections are to be completed in November.

President Barack Obama welcomed the U.S.-Russian agreement, calling it an "important, concrete step" toward the goal of destroying the weapons. He warned, however, that "if diplomacy fails, the United States remains prepared to act."

But the prospect of a U.S. military strike, which seemed just hours away only two weeks ago, now appears remote. The agreement suggests that a military strike could be authorized “in the event of non-compliance,” but that would come only after approval by the U.N. Security Council, where Russia holds a veto. In remarks before reporters here, Russian Foreign Minister Sergey Lavrov said such intervention was off the table for now.

There was no immediate reaction from Syria or Assad. In interviews on Russian television last week, Assad had said Syria would reveal the details of its chemical weapons stockpiles a month after its formal accession to the Convention on Chemical Weapons, the treaty that bans their existence. But it seemed unlikely that he would openly resist a timeline agreed to publicly by Russia, the principal provider of his military’s armaments.

Syrian opposition figures decried the agreement, saying it failed to hold Assad accountable for the deaths of hundreds of civilians in chemical weapons attacks. But defected Gen. Salim Idriss, who heads the U.S.-backed Syrian Military Council, said his group would “facilitate” the work of inspectors and would hold its fire when inspectors pass through government-held areas where rebels are fighting. He said under no circumstances, however, would the rebels observe a general cease-fire.

“In regions under our control, there are no chemical weapons,” he said. “I don’t know if this will just mean that inspectors will pass through the regions that are under rebel control. We are ready.”

Unknown was what the response would be of other rebel groups, including the Nusra Front and the Islamic State of Iraq and the Levant, both of which are affiliated with al Qaida and which comprise the Syrian rebels’ most effective fighters.

Magnus Ranstorp, an expert on terrorism at the Swedish Defense College, said in an email that both groups bear no love for the United States, Russia or the United Nations and could target the chemical weapons control effort, especially rebels from Central Asia and the Caucasus who have fought bitter battles against Russian influence in Chechnya and elsewhere.

“I think this is definitely a risk,” he wrote.

The agreement, spelled out in a four-page document that included two annexes, marked a remarkable development in the 30-month Syrian crisis that has been characterized not only by a brutal conflict that has killed more than 100,000 people from both sides and displaced 6.5 million from their homes, but also by a seemingly insurmountable impasse between the United States and Russia over how the conflict should be resolved.

Only two weeks ago, the United States appeared to be just hours away from sending cruise missiles into Syria in retaliation for an Aug. 21 alleged chemical weapons attack that killed hundreds of people, and Russia was staunchly defending its Syrian client, insisting that the rebels, not the government, were responsible. At one point, Russian President Vladimir Putin called U.S. Secretary of State John Kerry a “liar” and last week he wrote a column that appeared in the New York Times accusing the United States of violating international law for its Syrian threats.

Yet it took delegations from the two countries, led by Kerry and Lavrov, just two days to hammer out the agreement, and there was little sign of acrimony when the two officials announced the agreement Saturday.

The agreement itself also suggested that U.S. and Russian officials had shared intelligence with one another during the talks. The agreement says the two sides had agreed on estimates of the size of Assad’s chemical weapons stores. An official who asked to remain nameless because he was not authorized to discuss sensitive negotiations said the negotiators had concluded that that arsenal consists of about 1,000 metric tons of sarin, VX and mustard gas. A metric ton is about 2,200 pounds.

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Still, Kerry indicated there was a measure of U.S. mistrust, recalling President Ronald Reagan’s adage in dealing with the Soviet Union, “Trust, but verify.”

“That’s in need of an update, verify then verify,” he said.

Under the agreement, Assad has until next Friday to detail his country’s chemical weapons, “including names, types, and quantities of its chemical weapons agents, types of munitions, and location and form of storage, production, and research and development facilities.”

The agreement does not say what would happen if the Syrian government fails to hand over the list by Friday, though a senior U.S. state department official said that in itself could endanger Syria’s ability to sign on to the Organization for the Prohibition of Chemical Weapons, which is central to any U.N. agreement. The official spoke on condition of anonymity under State Department rules.

The agreement did not detail how precisely the weapons would be gathered for destruction, but said the two sides had determined that “the most effective control of these weapons may be achieved” by destroying them outside Syria.

Another U.S. official, who also asked to remain anonymous to discuss sensitive matters, said that in some cases the weapons could be rendered useless by quick means, such as burning the less dangerous alcohol-based part of a binary weapon. Syria is thought to maintain its sarin stocks in binary form. He noted that such work could very quickly remove the threat of parts of the arsenal.

As to where the chemicals should best be destroyed, an annex to the agreement said the countries would consider consolidating them in “coastal areas,” a possible reference to the Russian naval base at Tartus on Syria’s Mediterranean coast. Other materials would be either destroyed inside Syria or taken to other countries “depending upon site-specific conditions.”

Kerry said that the American assessment is that Assad’s regime has kept the chemicals under tight control and has moved them repeatedly to safer areas of the country, although Idriss said Saturday that his group has told the United States that it believes some of the weapons have been moved to Lebanon and Iraq.

The agreement said Syria has agreed “to provisionally apply” the ban even though its adhesion to the Convention on Chemical Weapons is not officially in force. Inspectors from the Organization for the Prohibition of Chemical Weapons “should be dispatched as rapidly as possible” and their initial on-site inspections should be completed by November, the agreement said.

November also is the date for destruction of “production and mixing/filling equipment.” “Complete elimination of all chemical weapons material and equipment” would take place “in the first half of 2014,” the agreement said.

The agreement makes no specific mention to the Aug. 21 alleged attacks that sparked the crisis. A U.N. report into that incident is expected to be released next week, perhaps as early as Monday.

That report is expected to provide a detailed description of what took place, including the kind of weapons used. While U.S. officials initially faulted the investigation for not taking on the task of laying responsibility for the attack, experts say that the details expected to be laid out will provide ample evidence of whether the government or rebels are responsible for the attack.

A person who took part in the negotiations and has studied what took place Aug. 21 said an unofficial estimate of the amount of chemical weapons deployed across Damascus’s eastern suburbs was “more than several hundred but less than 1,000 kilos,” or 2,200 pounds.

That would mean Assad maintains the capacity for between 1,000 and 3,000 attacks of a similar size until the Saturday agreement is carried out.

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